The damsel of Brittany. Eleanora, daughter of Geoffrey, second son of Henry II., King of England and Duke of Brittany. At the death of Prince Arthur she was the real heir to the crown, but John confined her in the castle of Bristol till death (1241).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Brittany from Infoplease: